A variety of different stored-value cards have been used as a substitute for cash payments. Magnetic stripe, microchip card, and capacitive cards have been used and each have their own unique properties and benefits. Capacitive cards and their corresponding read/write systems have been extensively described in previously published documents. For example, U.S. Pat. No. 3,699,311 issued Oct. 17, 1972 to Dunbar, describes a system that uses capacitive coupling to read binary data encoded as links embedded on a separate card. PCT application WO 84/00075 to Folkmann enhances the usefulness of the Dunbar concept by capacitively coupling high energy into a card to blow the embedded links thereby creating a write-once-read-many (WORM) card system. German patent 2,812,388 (Machate) describes a WORM capacitive card system that uses a capacitively-coupled write signal that achieves a high energy level by using a differential drive and a higher frequency than the read signal. There are many additional patents that are variations on the theme of capacitively-coupled data card systems, but these three are mentioned since they clearly identify some of the common and fundamental aspects of the technology.
All of the relevant patents describe a data card storage and retrieval system which includes an interface module and a data card each including a matching pattern of primary and secondary electrodes. The data card further includes a plurality of conductive links, each link extending between an isolated primary and a common secondary electrode. The "links" are referred to by various terms including connecting strips, data elements, bridges and fuses but their context clearly indicates that they refer to the same component in all cases. When a link is intact it is in a low-impedance state and when a link is broken it is in a high-impedance state. The state of each link is used to represent a binary bit of data.
In operation, the card is juxtaposed with respect to the interface module so that the matching electrodes are capacitively coupled. In order to read the data on the card, a signal having a predetermined voltage and frequency is directly applied to a selected primary electrode and the corresponding common electrode on the interface module. The signal is then capacitively coupled to the matching card electrodes. The signal then passes through the link which is directly connected between the card electrodes. The impedance of the link is determined and the result is used to derive the state of the link.
In those patents which also describe how information is written onto a card, the voltage level and/or the oscillating frequency of the impressed signal is increased causing a large current to flow in the link thereby blowing it. The hardware for a capacitive card system is generally low in cost, requires fairly low power to operate and provides a non-rechargeable or write-once-read-many (WORM) disposable memory card.
Being non-rechargeable, capacitive cards have a tremendous advantage over both earlier magnetic and microchip cards as they eliminate a high percentage of the least sophisticated but most common kind of fraud which is perpetuated by simply recharging otherwise legitimate cards. However, all earlier capacitive card systems have one major drawback in that it is difficult to protect against the production of counterfeit cards. This is primarily due to the relatively low memory density of the cards which is in turn due to the physical limitations on the number of electrodes that can be formed on a surface of a card.
The number of electrodes on the card is limited by the power available from the interface module. In many systems, particularly those relying on batteries for power, the available power is low. As the number of electrodes on the card are increased, their size must be reduced. As the electrodes are reduced in size, the amount of current that can be capacitively coupled to the electrodes is reduced. At some point, the electrodes will become so small that not enough current can be made to flow through the electrodes via the capacitive coupling to reliably fuse the links. Therefore, a data card of a given size can only accommodate a limited number of electrodes in a commercially viable system.
In all of the capacitive card systems referred to above, the design and arrangement of the primary electrodes, common electrodes, and fusible links limited the total number of data bits per card to the number of primary electrodes. Additional bits of data could be stored by using multiple links between each primary electrode and the common electrode. However, this approach does require a more sophisticated control scheme and much more sensitive and accurate read and write electronics. Any scheme with more than two or three fusible links between each primary and common electrode would be almost impossible to achieve in a commercially viable capacitive card system. In all of these prior art systems, there was no suggestion of an approach for providing additional data links by connecting a primary electrode to more than one other electrode in the pattern.
These restrictions severely limit the number of data bits which can be stored on a card. With only a limited amount of data storage, a card is relatively easy to counterfeit since it is not possible to utilize any security-enhancing encryption techniques. This weakness results in capacitive stored-value card systems having reduced market potential, as security of transaction has become the major consideration of most card issuers.
Using a uniquely different interface module and a new method of laser encoding the cards, both of which are described below, a significant improvement in counterfeit protection for capacitive card systems has been developed.
Accordingly, it is an object of the subject invention to provide a more secure capacitive data card system.
It is a further object of the subject invention to provide a new and improved method for manufacturing secure capacitive data cards.